The present invention pertains to an adduct of an epoxy compound and a cyclic phosphite, the use of such an adduct in a resin formulation, in synthetic materials, and in particular in prepregs and electrolaminates, and to the synthetic materials, prepregs, and electrolaminates thus obtained.
Phosphorous, flame extinguishing, curable epoxy resins are known per se. Swiss patent application 456,949 discloses that such epoxy resins are obtainable by adding to the formulation a particular quantity of adduct prepared from a 1,2-epoxy-containing compound and dialkyl phosphite or dialkenyl phosphite. Examples of dialkyl phosphite and dialkenyl phosphite are dimethyl phosphite, diethyl phosphite, dipropyl phosphite or dibutyl phosphite, as well as diallyl phosphite or dibutenyl phosphite. In the formation of adducts use is made of epoxy resins having, on average, more than one epoxy function per molecule.
U.S. Pat. No. 3,971,813 describes adducts of hydantoin-containing epoxy resins and phosphites. These epoxy resins contain two or more glycidyl groups per molecule. Specifically, only acyclic phosphites are disclosed in this patent specification (i.e. dimethyl, diethyl, and di-n-butyl phosphite). The drawback to the examples described in this patent specification is that the adducts obtained are not in the pure form, and that volatile components are formed which have to be removed under low pressure on conclusion of the reaction.
The use of the dialkyl phosphite compounds mentioned in the examples results in the formation of oligomeric products of high viscosity. Said high viscosity is the result of the dialkyl phosphite compound being capable of both adding to an epoxy group and transesterifying with two hydroxy groups, which creates very large, branched molecules and may lead to gelling. The alcohol and any residual dialkyl phosphite compound have to be removed from the resulting crude adduct at elevated temperature and reduced pressure. The removal is especially hard when the resin is viscous or the alcohol is high-boiling. Residual alcohol is disadvantageous also because during the curing process, particularly at high temperatures, it will cause the release of yet more alcohol from the chemically bound alkoxy groups, which will interfere objectionably with the curing process or lead to blisters in the cured product.
Polymers prepared from compounds carrying two a-epoxy groups with spiro phosphites, such as 3,9-dioxo-3,9-diphospha-2,4,8, 10-tetraoxa-spiro[5,5]-undecane, have been disclosed in Japanese patent publication Sho 46-20824. However, the products concerned are high-molecular polymers which do not contain epoxy groups, and which are not cured and used for further processing into products with extra epoxy groups for application in electrolaminates.